FIG. 7 shows the configuration of a PWM signal generating section of a PWM inverter in a first conventional technique.
In FIG. 7, numeral 3 denotes a comparator. Symbol a denotes a signal wave, b denotes a carrier wave, and PWM denotes a PWM signal.
As shown in FIG. 7, the PWM signal generating section of the conventional PWM inverter compares a signal wave a and a carrier wave b to determines an output state according to their amplitude magnitude relationship (see e.g., Patent document 1).
FIG. 8 shows a PWM signal waveform in the PWM signal generating section of the first conventional technique. In FIG. 8, symbol a denotes a signal wave; b, a carrier wave; t, time; T, a period of the carrier wave b; and PWM, a PWM signal.
The PWM signal waveform in the first conventional technique will be described below with reference to FIG. 8.
If, for example, the PWM signal is given a value “1” when the signal wave a is larger than the carrier wave b and is otherwise given a value “0”, a PWM waveform as shown in FIG. 8 is obtained. In this case, if the amplitude of the carrier wave b is assumed to be “1,” average values, over one period T of the carrier wave b, of the signal wave a and the PWM signal are approximately the same.
FIG. 9 shows the configuration of a PWM signal generating section of a PWM inverter in a second conventional technique. In FIG. 9, numeral 3 denotes a comparator for comparing a signal wave and a carrier wave and numeral 4 denotes a trigonometric function calculating section. Symbol A denotes a desired amplitude; Θ, a desired phase; a1, a signal wave; b, a carrier wave; and PWM, a PWM signal.
FIG. 10 shows a PWM signal waveform in the PWM signal generating section of the second conventional technique. In FIG. 10, symbol a and a1 denote signal waves; b, a carrier wave; t, t1, and t2, time and time points; T, a period of the carrier wave b; and PWM, a PWM signal.
The operation of the PWM signal generating section of the PWM inverter of the second conventional technique will be described below with reference to FIGS. 9 and 10.
With the progress of the digital control in PWM inverter, in the case where vector control or the like is used, a signal wave a is generated frequently (commonly) using a trigonometric function on the basis of two data, that is, an amplitude A and a phase θ. In this case, a signal wave a1 is given by Equation (1):a1=A sin θ  (1)
Because of the digital control, Equation (1) is executed in a certain control cycle. For example, in the case where it is executed in synchronism with a carrier wave frequency, the signal wave a1 is generated by quantization whose period is equal to the carrier wave period T. As a result, the signal wave a1 is given a waveform as shown in FIG. 10. The comparator 3 performs comparison using such a signal wave a1.
As described above, in the PWM signal generating section of the conventional PWM inverter, digital control is used in generating a signal wave a1 from two data (amplitude A and phase θ) using a trigonometric function. The signal wave a1 and a carrier wave b are compared by the comparator 3 to determine an output state according to their amplitude magnitude relationship.
Patent document 1: JP-A-2000-102257 (FIG. 3)